FRIDAY, Jan. 12, 2018 (HealthDay News) -- Wildfires like those that recently burned wide swaths of California to the ground can cause air quality problems that pose a threat to human health far from the fire site, researchers say.

An analysis of U.S. government data collected from 2006 to 2013 found that smoke plumes from wildfires can be carried high into the air and spread over thousands of miles -- even days after a fire has been extinguished.

The fine particles and ozone contained in these smoke plumes can significantly reduce air quality in cities, according to the researchers. This puts residents at increased risk for health issues such as breathing and heart problems, the study authors said.

Compared with clear days, ozone concentrations were about 11 percent higher and fine particle levels 33 percent higher on days when wildfire smoke plumes were present, according to the report.

Wildfire smoke plumes occurred on just 6 percent to 7 percent of days in the study period. However, the plumes accounted for 16 percent of unhealthy days due to small particles and 27 percent of unhealthy days due to ozone, the findings showed.

The findings were published online recently in the Journal of Exposure Science and Environmental Epidemiology.

"Smoke-plume days accounted for a disproportionate number of days with elevated air quality index levels," study author Alexandra Larsen, from North Carolina State University, said in a journal news release.

This indicates "that moderate increases in regional air pollution due to large fires and long-distance transport of smoke can tip the air quality to unhealthy levels," Larsen said.

"Enhanced ozone production in urban areas is a concern because of the population size potentially impacted and because air pollution levels could be already elevated due to local and mobile sources," she said.

According to the researchers, large-scale wildfires -- those affecting 10,000 acres or more -- have increased fivefold in the United States since the 1970s.